A feature of sealed ESDs is containment and isolation of the cells within the ESD from external contamination and/or interference. Conventional containment devices (e.g., ESD wrappers) provide containment and isolation for the various phases of the ESD materials, such as the electrolyte solvent mixture and the active materials. It is often necessary to contain these ESD materials while the cells are in rest or in operation. While the cells are in rest or in operation certain materials contained within the cell experience changes in state. The fluctuations in state result in variations in cell pressure which impart stresses on the containment device.
Pressure within a fixed volume container is proportional to the amount and state or states of the active materials within the container. For example, in metal hydride ESDs the electrochemical couple includes a metal hydride alloy within the anode and a metal hydroxide within the cathode. Each of these materials undergoes state changes which result in a net volumetric shift within the container. The volumetric shift may result in changes in cell pressure and/or net changes in free volume. Therefore, active material state changes can result in less, or more, available free volume, where free volume refers the difference in the unit cell volume from the volume of all of materials contained within the cell. In the exemplary metal hydride configuration, hydraulic pressures from displaced liquids, gas phase components of the electrolyte solvent mixture, and partial pressures of the vapor phase of the electrolyte solvent mixture generate forces against the container walls. The pressures attributed to the changing active material states also cause electrode surfaces to physically press against container walls. If the stress exceeds the capability of the containment device, the device may be compromised and may not function properly.
Conventional ESDs have been manufactured as either a wound cell ESD that has only two electrodes or a standard prismatic cell ESD that has many plate sets in parallel. In both of these types, the electrolyte is shared everywhere within the ESD. The wound cell structure and prismatic cell structure both employ rigid, fixed volume containment. These structures require strict accounting of the materials within the rigid container because the container does not allow volumetric expansion and/or contraction (i.e., the amount of free volume is fixed). The amount of free volume may affect the operation of the ESD as too much free volume results in poor performance (e.g., a loose cell) and too little free volume results in a potentially explosive device. In some instances the forces generated during operation of these conventional ESDs may exceed the material limits of the container and cause the container to rupture. When the container ruptures due to high pressures, the internal constituents are ejected with a significant amount of energy.
Accordingly, it would be desirable to provide an ESD having variable volume containment.